This invention relates to a precalciner for cement raw meal.
Various types of cement production equipment including a precalciner provided with an independent heat source and interposed between a suspension preheater and a rotary kiln have been proposed and the equipment is characterized by the types of its precalciner.
Among the cement production equipments, one in which a spouted bed of cement raw meal is formed by kiln exhaust gas for calcining the raw meal has such advantages that (1) the construction of the equipment is simple, (2) the pressure loss is relatively small, (3) since the raw meal is circulated in the spouted bed, the residence time of the raw meal is relatively long, and (4) since the kiln exhaust gas duct is connected to the bottom of the precalciner, such foreign matters as coatings created in the precalciner or fallen from a preheater during the operation of the equipment can be discharged into the kiln.
FIG. 1 shows one example of the prior art cement production equipment of the type described above which comprises an induced draft fan 1 for exhausting of the entire gas, cyclones 2 through 5, a raw meal feed pipe 6, a precalciner 7, a kiln exhaust gas duct 11, a burner 9, a raw meal feed pipe 8 for the precalciner 7, a tertiary air duct 10 for tertiary air extracted from a clinker cooler 13, a rotary kiln 12, and a raw meal feed pipe 15.
Elements bounded by dot and dash lines constitute a preheating device P.
The upper end of the precalciner 7 is connected to the lowermost cyclone 5 via a precalciner exhaust gas duct 14, while the lower end is connected to the kiln exhaust gas feed duct 11.
The bottom of the precalciner 7 has an inverted cone shape so that the kiln exhaust gas forms a spouted bed and the upper portion of the precalciner 7 is cylindrical.
It will be understood by those skilled in the art that the cyclones of the preheating device P may be replaced with known counterflow type heat exchangers.
With this construction, the major portion of the raw meal fed into the precalciner descends along the wall surface thereof and then is blown upwardly from the bottom of the inverted conical part by spout of the kiln exhaust gas. A portion of the raw meal blown upwardly by the spout is brought to the upper portion and exhausted from the precalciner, while the remaining portion diffuses toward the wall surface of the precalciner and again flows down therealong. Accordingly, a circulating flow of the raw meal is created to increase the residence time of the raw meal in the precalciner. This phenomenon is enhanced by introducing the tertiary air as a swirling flow. The necessary residence time can be maintained by the circulation of the raw meal.
The prior art precalciner as shown in FIG. 1 having an inverted conical bottom and a cylindrical upper portion has the following disadvantages. More particularly, in order to ensure a sufficiently long residence time of the raw meal in the precalciner, it is necessary to increase the ratio between the diameters of the cylindrical portion and the outlet throat of kiln exhaust gas so as to increase the circulating load of raw meal. Consequently, the circulation phenomenon of the raw meal mainly concentrate at the lower inverted conical portion, thus greatly increasing the concentration of the raw meal at this portion and decreasing the concentration at the upper portion.
Then, a stagnant area of the raw meal is created in the inverted conical portion. Under normal state, this stagnant area balances the spout, but when the draft varies, the raw meal concentrates at the kiln exhaust gas feed port to amplify the draft variation and to cause the not yet calcined raw meal to fall into the kiln.
Moreover, since the combustion region of the fuel is close to the bottom of the cylinder, it is necessary to maintain the raw meal concentration at this region at a high value in order to perform efficient calcination.
With the construction described above, however, the raw meal concentration at the inverted concial portion is extremely high while low at the cylindrical portion, so that it is difficult to maintain a balance between the heat of combustion and the heat of calcination, thereby forming a local high temperature region which tends to cause a coating trouble.
Moreover, in the bottom raw meal stagnant area, since the raw meal is not sufficiently admixed with the gas, heat transfer therebetween is poor, thus decreasing reaction rate of calcination in this region. For this reason, even with the same mean residence time, the effective residence time becomes shorter as compared to a case wherein the dispersion of the raw meal is sufficient. Consequently, the calcination rate of the raw meal decreases. To increase this rate, it is necessary to increase the volume of the precalciner.
The localization of the raw meal at the inverted conical portion may be avoided by decreasing the cone angle of the inverted conical portion but such a construction increases the height of the precalciner.
Moreover, when the cone angle of the inverted conical portion is decreased, the speed of falling down of the raw meal along the wall surface increases, thus weakening the function of the kiln exhaust gas for blocking the raw meal so that the not yet calcined raw meal tends to fall into the kiln. To prevent this phenomenon, it is necessary to greatly increase the spouted flow velocity of the kiln exhaust gas, resulting in an increased pressure loss.
This construction increases the gas speed at the lower portion of the inverted conical portion, so that the concentration of the raw meal decreases greatly, with the result that the local high temperature region increases, thus causing the trouble of coating.
As described above, with the prior art precalciner, it has been very difficult to simultaneously satisfy such requirements as attainment of residence time of the raw meal, uniformity of the concentration distribution of the raw meal, attainment of proper concentration of the raw meal, attainment of proper height of the precalciner, and reduction of the pressure loss.